johnson



Aug. 11, 1959 l H. R. JOHNSON BACKWARD-WAVE OSCILLATOR 2 Sheets-Sheet 1 Filed Aug. 25, 1954 9 Allll Aug. l1, 1 959 y H. R. JOHNSON 2,899,594

BAcKwARD-WAVE SCILLATOR- Filed Aug. 25, 1954 2 Sheets-Sheet 2 2in1/c @im .55mm/fri /a//af i wwwa/V, y

United States Patent() 'z,s99,s94 BACKwARD-WAVE osclLLAToR Horace R. Johnson, Venice, Calif., assignor to Hughes Aircraft-Company, Culver City, Calif., a corporation of Delaware f Application August 2'5, 1954, Serial No.452,1-91 9 claims. (cialis-3.5)

This invention relates to improved backwarddwave oscillators and more particularly to a method of and apparatus for eliminating'v changes in the frequency of oscillation of a backward-wave oscillator' normally accompanying' variations in the output load impedance. g

As is presently known, a backward-wave oscillator may comprise a traveling-Wave amplifier tube wherein the velocity of its electron stream is synchronized so as to amplify a wave which has a group velocity in a direction opposite to the directionof electron oiw. A characteristic of this type of oscillator is that it may be electronically tuned over a wide range of frequencies by merely varyingV the velocity of the electron stream. A description of a baclcward-wrotey oscillator of this general type appears in a copending application for patent, Serial No. 371,796, entitled, Backward-Wave Oscillator tiled on August 3,v 1953', by Dean A. Watkins, now abandoned".v

"CC v a backward-wave oscillator incorporatingapparratus -to' progressively decrease synchronsr'nA between the electron stream and the electromagnetic wave propagated byV the slow-wave structure throughout the' portion thereofadja centi the collector end of theftube. Y

The novel features which are believed to he character; istie of the invention, both as' to its organization` and method of. operation, together withnfurther objects and advantages thereof, willl be better understood. frontthe i' following description considered in connection with the accompanying'` drawings inwhich several? embodiments of the invention are illustrated by way ofe'xarnplet` Iii-'is' to be expressly understood, however, that-.the drawings are for the purpose of illustration and description only',

l and are not intended` as a definition of the vlimits of' e The output wave from backward-wave oscillator tubes of the aforementioned type has been found to experience substantial fluctuations in the power output and considerable frequency pulling, i.e. changes in' the oscillating frequency due to variations in the output load impedance'.

It is therefore an object of the present invention to provide a method of and apparatus for reducing frequency pulling and power output uctuations in backward-wave oscillator type tubes.

It has been found that these detrimental effects are caused by successive primary and secondary energy reflections from the extremities of the slow-wave structure nea-rest the gun end and the collector end of the tube. Since the amount of primary reliection at the gun or outputl extremity of the tube depends upon the manner in which the tube is used, iti remains to minimize the secondary reflection at the collector extremity. In accordance with the present invention, this isa'eeor'npl'ished` b'y disposing means for attenuating electromagnetic wave energy propagated by the slow-wave structure along the portion thereof adjacent the collector end of the tube.VV Coextensive with this portion ofy the slowawave' structure', synchronism between the electron stream and the electromagnetic wave is progressively decreased so as' to prevent additional energy from being transferred from the electron stream to the reilected wave. Thus, along this portion of the slow-Wave structure there is substantially no interaction between the slow-wave structurel and the electron stream. During operation of the tube, the electromagnetic wave energy reflected from the output load impedance of the tube and propagated back towards the collector end of the tube by the slow-'wave struct-ure is rst attenuated along the portionthereof adjacent the collector end of .the tube. Secondly, .wave energyy res ilected from the collector end of the tube is likewise attenuated along the same portion of the slow-Wave structure rather than being amplified bythe transfer of energy from, the` electron stream as' is presently the case. Thus, the device of the present invention incorporates structure capable of attenu'ating'lwave energy reflected from the load coupled to the tube' thereby substantiany'renneing Y invention. A

Fig. l is a diagrammatic sectional view o f' an embotliI mentr of the invention and associated electric circuits",` Fig. 2 is a cross-sectional view along section L-Z ot Fig. 3 is an enlarged perspective'view .of ari-extremity of a helix supporting rodl showing the conguration of a resistive coating; Fig. 4 is a graph'v illustrating the variation of. an electric held across the enlarged size lhelix for the fundamental and backward-vvave modesof propagation; and i Figs. 5 and 6 are partial sectional views of alternat'eema bodiments of the present invention'. 1 Referring now to Fig.l, there is; shown' dia'grarnniatiotl 1y a backward-wave oscillator of the present invention; An envelope 10, which provides thel necessary evacuated chamber, consists of a long cylindrical portion llvand an enlarged portion 12 at thel left extremity asV Viewedtnfthe drawing. Contained within-'the enlarged 4portion-1220i envelope 10 there is disposed an electron gun 14.for'pro-' ducing and launching an electron stream alongaa hollow cylindricalA path disposed concentrically about kthe lungi tudinal axis of envelopel'. t Electron gun 14 comprises an annular cathode 15 with a heating element' lo,` a focusa ing electrode 18, and an accelerating anode' 20, the electrodes 18 and 20 being provided with annular apertures in register with the electron emittingsurface of cathode 15 to allow passage therethrough of the electron strearr'r.y Cathode 15 is disposed concentrically about the longi# tudinal axis of envelope 10 withits electron ernissivef'sur face facing the'long cylindricalA portionv 11. Its heating element 16 is connected across a source of potential; such as a battery 22, one terminal of Which'may be lcon-- nected to cathode 15, as shown. The electrode .18 pro vides an inner and an outer conductivels'urface of lrevolud tionV disposed about the longitudinal axisv of envelope 10 and adjacent the electron emissive surface of cathode 15 as shown in the gure.- These inner and outer conduca tive surfacesV of revolution. are positioned atanangle of approximately 67.5 with the direction of electron-How? and are connected4 to'gathoderl to provide focusing for the electron stream. Cathode 1.5 is, inturn, c onnectedftov an adjustable tap 24 of a potentiometer 26,.wh ich iis connected across a battery 2.8, the positiveterminalr of which is connected toY ground.y The magnitudefof the` 3 viewed in the drawing. Anode 2 0 is maintained at a substantially tixed potential that is positive with respect to the potential of cathode 15 such as, for example, ground potential. This may be accomplished by means of grounding the anode 20. j f Positioned concentrically aboutand contiguous to the path vof the electron stream in the direction of electron ow is a conductive ribbon helix 36.V 'Ihe circumference of the helix 36 is distinguished from conventional sized helices in that it isv comparatively large with respect to a free space wavelength at the operating frequency of the tube such as, for example, one-half a free space wavelength. In general, this necessitates that the inner diameter of the hollow stream be at least 0.7 the inner dialneter of helix 36 in order that the helix present a comparatively high impedance to the electron stream. A material such as tungsten or molybdenum is suitable for making the helix, the main prerequisite being that it retain its form, especially with respect to its pitch and diameter. l The helix 36 is retained in position by three dielectric rods 38, 39 and 40 disposed at equal intervals in the intervening space between the helix and cylindrical portion 11 of envelope 10 as shown particularly in Fig. 2. These dielectric rods may, for example, be composed of zircon or Synthetic sapphire. A termination for electromagnetic wave energy propagated by the helix 36 is provided by resistive coatings 42, 43 and 44 which are disposed on the side of dielectric rods 38, 39 and 40, respectively, in contact with the helix 36 along the last several turns thereof farthest from the electron gun '14. The resistive coatings may be provided, for example,'by spraying a colloidal suspension of graphite commonly known as' Aquadag" on the rods. The general configuration of the resistive coating on the dielectric rods is shown in Fig. 3 wherein the resistive coating 42 on the rod 38 is illustrated. In this ligure, a dashed line 45 represents the line of contact betweenthe rod 38 and the helix 36. In order to make the transition in the impedance from the unattenuating portion to the attenuating portion of the helix 36 as gradual as possible, a triangular shaped mask is employed in spraying on the graphite suspension so that the coatings, in progressing towards the end of the rod, commence at points 46, 47 on the side of the rods inquadrature from the line andextend inwards towards the line 45 until they meet at a point 48. The entire one side of the rod is covered with the resistive coating between the point 48 and the extremity of the rod on the side thereof in contact with the helix 36.

`A section of coaxial cable 50 provides an output from the extremity of helix 36 adjacent to the electron gun 14, The center conductor 51 of coaxial cable 50 connects the output terminal through the envelope 10 to the end of the irst turn of helix 36. The outer conductor of cable 50 isconnected to a ferruleV A52 which is disposed concen# trically about the first several turns of helix 36 on the outside of envelope 10. The extremity Vof ferrule 52 .farthest from electron gun 14 is flared out in order to improve the impedance match between coaxial cable 50 and helix 36. The outer conductor of cable 50 and ferrule 52 may be maintained at any substantially fixed potential such as, for example, at ground by means of a connection thereto. In the operation of the backwardwave oscillator of the present invention, it is intended that the output terminals be connected across a load impedance such as a resistor 54. In this manner, a resistive path from helix 36 to ground is provided therebymaintaining it at quiescent ground potential. Y

- An equipotential region is provided between the accelerating anode 20 and helix 36 by means of aconductive coating 56 disposed about the inner surface of envelope V10 therebetween. Conductive coating. 56 may be provided by a colloidal suspension ofgraphite in the same manner as the resistive coatings 42, 43 and 144 on the dielectric rods 38, 39 and 40. Conductive coating 56 '4 is maintained at the same potential as that of accelerat ing anode 20 by means of a cpnnection directly to ground.

Positioned concentrically about the complete length of the envelope 10 is a solenoid 60. An appropriate direct current is made to ow through solenoid 60 by means of connections across a battery 62 so as to produce a magnetic field of the order of 600 gauss which initially extends axially along the entire length of the tube. The purpose of the magnetic eld thus generated is to direct the hollow 4cylindrical electron stream launched by the gun 14 along a path continguous to helix 36.

At the extremity of helix 36 farthest from electron gun 14 there is disposed a collector electrode 66 to intercept and collect the electron stream. Collector electrode 66 comprises a nonmagnetic metallic portion 68 constituting a metallic disc of substantially the same diameter as the portion 11 of envelope 10 and having two concentric annular extensions 70 and 72, the extension 70 being from its outer periphery and the extension 72 being from a diameter of the order of 0.6 the inside diameter of the helix 36. The extension 72 is several times the pitch of helix 36 longer than the extension 70. In addition, the collector electrode 66 includes a magnetic material 74 disposed within the cylindrical container provided by ex tension 72 from the metallic disc portion 68. Electrode 66 is mounted by sealing the portion 11 of envelope 10 to the metallic extension 70 as shown in the gure thereby mounting the extension 72 concentrically within the last several turns of helix 36. Also, in order to prevent secondary electrons ejected from the collector electrode 66 by the electron beam from being attracted back to the helix 36 when operating the tube, the collector electrode is maintained at a potential of the order of 200 volts positive with respect to the potential of the helix 36. rIhis is accomplished by means of a connection therefrom to the positive terminal of a battery 76, the negative terminal of which is connected to ground.

In operation of the tube of the present invention, the electron stream is directed contiguously along the inner surface of the helix 36 for its entire length except for the portion adjacent the collector electrode 66 coextensive with the resistive coatings 42, 43 and 44 of ceramic rods 38, 39 and 40. In the region thus occupied by the electron stream, the forward and backward-wave impedance offered by the helix to the stream is comparatively large. In a relatively large helix having a circumference of lthe order of one-half the free space wavelength at a desired operating frequency, the axial electric eld components of both forward and backward waves are a maximum at the periphery of the helix and decrease rapidly in both inward and outward radial directions. The configuration of the axial electric field components of forward and backward waves of a large-size helix of the type described above is illustrated in Fig. 4. In this ligure, the vsolid lines show the variation in the axial electric field components of a forward wave versus radial distance from the center line of the helix. Similarly, the dashed lines 82 show the variation in the axial electric eld components of a backward wave propagated by the helix. In the case of a large helix of the type herein employed, the axial electric components of a forward wave are just slightly larger and of the Same general configuration as those of a backward wave as indicated in Fig. 4.

As is generally known, the degree of interaction between an electromagnetic wave propagated by the helix and an electron stream is dependent upon the impedance presented to the electron stream for the` particular mode being propagated. More particularly, the impedance offered by a helix to an electron stream is distinct for each mode of propagation on the helix and is primarily a function of the relative intensity of the axial electric eld components existing in the region occupied by the electron stream. Thus, for the backward-wave mode of propagation, the impedance presented to the electron stream is at its maximum in the region nearest the periph- 45 ery of the helixl and'approaches zero inthe'regions vat radii less than one-half the radius of the helixe i Y, Inthat the backward mode of propagation is inherently regenerative in eiect,A a very smallstream current is required for oscillations to commence. This is. so be cause the group velocity, i.e.` the velocity of energy olw, of a backward wave is towards the source of the stream electrons. Thus any initial perturbation of the electrons of the stream is constituted of frequency components including the frequency at which oscillations will occur. Electromagnetic energy constituted of these frequency components is` amplified and propagated towards theelectron gun 14. This energy modulates the electrony stream, which modulations are, in turn, propagated towards the collector electrode 66 during which time they induce Wavesl onthe helix 36. The Wave is in phase with a wave initially impressed on the helix, 'will grow in amplitude. It is evidentthat the frequency of the wave which grow in amplitude, i.e. the frequency of oscillation, is dependent on the velocity of the electronlstrcam as the stream velocity is essentially the only variablev in the feedback loop.

Therefore it is onlynecessary to develop an electron stream of suitable amplitude and directl it along the helix to produce backward-wave oscillations. At the extremityof the helix 36 nearest the electron gun 14, thebacltward wave is transferred to the output coaxial cable 50 Yand subsequently impressedA across theload impedance represented by the resistor 54. vThe frequency of oscillation is controlled by varying the voltage between cathode and helix 36. In the present case,` this is accomplished by adjusting the position of tap- 24. of potentiometer 26.. It is evident that the output energy` may also be frequency modulated by impressing a modulation signal on the potential of cathode 1.5. I A

. Because of the wide range over which thefrequency of theqoutput energy may be electronically. tuned, it is `generally rather diicult to obtain impedance matching between the helix 36 and the output circuit.` This results in a portion of the output energy being reected near the gun and propagated back towards the collector electrode 66 by the helix 36. Under normal conditions, the frequency of this reflected energy is such that .all of its wave components have phase velocities that are outside of the range necessary for synchronism lwith the electron stream. Thus, the energy reected from output circuit is propagated by the helix 36 to the collector end of the tube.

As previously set forth, in connection with. Fig. 4 the axial electric fields of a forward wave exist principally about the periphery of the helix 36. Thus as theforward wave passes the commencement of the resistive coatings 42, 43 and 44 of ceramic rods 38, 39 and 40 there is very little effect as the coatings begin at points on the rods in quadrature from the line on the rods contacting the helix. The thickness and density of the resistive coatings is likewise gradually tapered. As the waveV progresses, however, the resistive coatings approach the yperiphery ofthe helix and increase in thickness and density to provide maximum attenuation. The purpose of this gradual increase in attenuation is to prevent any of the forward Wave energy from being reflected, as any energy reected back towards the gun 14`Will be amplified as a backward wave. v I

ven with the resistivecoatings 42, 43 and 44, itis generally not possible to attenuate all of the forwardwave energy or to provide a perfect impedance match at the collector. end of helix 36. As mentioned above, there isa tendency for any-reflected energy to be'amplied as a backward wave. In accordance with the present invention, complete attenuation of the forward wave initiated by a portion of the output energy being reflected from the output circuit is obviated by eliminating synchronism between the stream and the backward waves along the portion of the helix 36 adjacent the collector 66. yImthe embodiment oflig. i., thisv is accomplished; by themagneticgelement 7410i'` collector electrode-66. The element 74, has a lower reluctance magnetic path as cornpared to the relatively high reluctance of the surrounding vacuum and causes the axial magnetic field to converge and Athread theelement 74. The electron stream follows the magnetic lines of forceeandthus is progressively directed away from the inner periphery of the helix 36 resulting in decreasingly less interaction between the electrons-of the stream and thewave propagated bythe helix 36 throughout this portionof the path. At the end of the path, the electrons of the stream are intercepted and collected by themetallie extension 72 of collector electrode 66. Therefore, in view of theabove, it is evident that any energy of a forward wave that is not dissipated in the resistivecoatings 42, `43 and 44 and thatmay again be neected will continue tobe dissipated in the resistive coatings without being ampliried.

Apartial sectional" view of an alternative embodi- Anientpfl the tube of the present `invention Ais shown in Fig. 5 This embodiment is particularly adapted to the modification of existing tubes to operate in accordance with the present invention. The embodiment of Fig. 5 isthe sante as shown and described inconnection with Fig. l exceptfor an auxiliary solenoid anda conventional cup-shape collector electrode 94. Auxiliary solenoid 90 is disposed concentrically aboutr thecylindrical portion 11 of envelope `140 coextensive with the resistive coatings 42, 43 and 44 on ceramierods 38, 39 and 40. Solenoid 90 is energized by suitable connections across `a battery 91 soas to inerease the axial magnetic `field vthroughout its length. This increased axial magnetic field converges the hollow cylindrical electron stream so that its path gradually bends inwardsfrom th'e periphery of helix 36 as illustrated. The operation of the tube isthesarneasthatofFigl.. Y f

A vthird embodiment ofthe tube of the .present linvention illustrated in Fig.y 6 incorporates a helix 96 having either a progressively larger or a smaller pitch along the portion of its length that is coexten'sive with coatings 42, 43 and 44 of ceramic rods 38, and 40. Fig. 6 illustrates a partial sectional view of this embodiment showing a helix 96 having a decreasingly smallerv pitch. In this embodiment, the pitch of the helix96 is -such that the velocity of a backward wave propagated by the helix along the portion of its length coextensive with the resistive coatings 42, 43 and 44 is' not in synchronisrn with the electrons of the stream.4 Thus, the operation of this embodiment is lthe same as the prior embodiments illustrated in Figs.' l and 5 intthat-there is no amplificationl of the energy reected from the collector end of the helix '.coextensive with the resistive coatings, 43 and 44. A- feature of this embodiment is that the spatial relationship between the electron stream and the helix 96frer`nains-the same. throughout the entire` length of the helix.v vThis makes it possible to maintain constant the impedance presented to the electron stream by the helix along the length of stream.

What is claimed is:

1.--A backward-wave oscillator comprisinga recurring slow-wave structure capable of propagating a backward wave along a predetetrnined path therethrough, means for producing an electron stream, means for directing said electron stream along said=predetertnined path to generate 'a backward. wave on.said'recurn'ngf-v structure of a fre'- quency' substantially proportional tothe velocity of said electron stream, output means`r coupled tosaid recurring structure at the extremity thereof iirst'entered by said electron stream for making said backward wave available as an output signal from said oscillator, whereby a portion of the energy of said output signal is undesirably reflected and propagated towards the other extremity of said recurring structure, attenuating means for attenuating the rellected energy of said output signal along a portion of the length of ,said predetermined path adjacent said otherextremity whereby a residualportion of the lenergy of said output signal is undesirably again reflected, and synchronous interaction limiting beam deecting means for substantially preventing transfer of energy from said electron stream to said reflected residual portion along said portion of the length of said path adjacent "sfaid, otherextremity, thereby enabling the energy of said reected residual portion to be dissipated by said attenuating means.

2. A backward-wave oscillator compnsing a conductive Vhelix having a predetermined diameter, means for generating a hollow cylindrical electron stream having a diameter smaller than said predetermined diameter, means for directing said hollow electron stream concentrically through said helix to generate a backward wave thereon of a frequency substantially proportional to the velocity of said electron stream, output means coupled to said helix at the extremity thereof first entered by said electron stream energy of said output signal is undesirably reflected and propagated towards the other extremity of said helix, attenuating meansI for attenuating the reflected energy of 'said output signal along a portion of the length of said venting the synchronous coupling of energy from said electron stream to said reflected residual portion along said portion of the length of said helix adjacent said other extremity, thereby enabling the undesired energy of said 'refiected residual portion to be dissipated by said attenuating means.

3. YThe backward-wave oscillator as defined in claim 2 ,wherein said conductive helix is supported by a plurality of ceramic rods disposed at intervals around the periphery thereof, and said attenuating means includes a Yresistive coating on at least one of said ceramic rods disposed therealong coextensive with said portion of the length of said helix adjacent said other extremity.

4. The backward-wave oscillator as defined in claim 2 fwherein said means for substantially preventing the trans- -fer of energy from said electron stream to said reflected residual portion includes means for directing said electron stream away from the periphery of said helix.

5. The backward-wave oscillator as defined in claim 2 wherein said means for preventing the transfer of energy from said electron stream to said reliected residual 'portion includes means forpropagating said reflected residual portion along said portion of the length of said helix at a velocity that is different from the synchronous velocity of said electron stream.

6. A backward-wave oscillator comprising a conductive :helix having a predetermined diameter, an electron gun 4disposed at one extremity of said helix for generating a hollow cylindrical electron stream having a diameter of at least 0.7 of said predetermined diameter, an element of 'a material having a permeability greater than unity disposed within the opening of said helix at the other extremity thereof, means for producing a magnetic field alongV the length of said helix for directing said hollow :electron stream therethroughto generate a backward wave vthereon, whereby said magnetic field converges toward 4said element to increase the distance between the path :of said stream and the periphery of said helix along a portion of thelength thereof adjacent said element, output ,means coupled to said helix at said one extremity for 'for making said backward wave available as an output 4signal from said oscillator whereby a portion of the making said, backward wave available as an output signal fromA saidoscillator, whereby a part of the energy of said output signal is Vinherently and undesirably reflected and propagated by said helix towards said element, and attenuating means disposed contiguously along said portion of thev length of said helix for substantially dissipating said part'of the energy of said output signal.

7. A backward-wave oscillator comprising -a conductive helix having a predetermined diameter, an electron gun disposed at 'one extremity of said helix for generating a hollow cylindrical electron stream having a diameter of at least 0.7 of said predetermined diameter, means for producing afirst magnetic field along a portion of the length of said helix adjacent the other extremity thereof, means for producing a second magnetic field along the entire length of said helix for directing said hollow electron stream ltherethrough to generate a ,backward wave thereon, whereby, the combined magnetic field constituting said first andA second magnetic field directs the electrons of said stream away from the periphery of said helix along said portion of the length thereof adjacent said other extremity, output means coupled to said helix at *said one extremity for making said backward wave available as an output signal from said oscillator, whereby a part of the energy of said output signal is undesirably reected and propagated by said helix towards said other extremity, and attenuating means disposed contiguously along said portion of the length of the helix for substantially dissipating said part of the energy of said output signal.V

8. A backward-wave oscillator comprising a conductive 'helix of predetermined diameter, said helix having a uniform pitch along a major portion of its length and a different pitch yalong the remaining portion of its length, an

electron gun disposed at a first extremity adjacent said major portion of the length of said helix for generating a hollow cylindrical electron stream having a diameter of at least 0.7 of said predetermined diameter, means for ldirecting said electron stream concentrically through said .means disposed contiguously along said remaining portion of the length of said helix for dissipating said part of the energy of said output signal.

9. The backward-wave oscillator as defined in claim 8 wherein the pitch of said helix is increasingly smaller than said uniform pitch along said remaining portion of its length.

References Cited in the le of this patent UNITED STATES PATENTS 2,575,383 Y. Field Nov. 20, 1951 2,702,370 Lerbs Feb. 15, 1955 2,707,759 Pierce May 3, 1955 2,794,143 Warnecke et al May 28, 1957 2,812,467 Kompfner Nov. 5, 1957 FOREIGN PATENTS 699,893 Great Britain Nov. 18, 1953 `1,080,027 Y France May 26, 1954 OTHER REFERENCES Article by E. Dench, pages 64 to 66 and 157 to 162, Tele-Tech and Electronic Industries for November 1953. 

